1. Field of the Invention
The present invention relates to a method for generating three-dimensional form data having a small amount of data than the original three-dimensional form data and an apparatus therefor.
2. Description of the Prior Art
Active stereo methods are used for measuring a form of an object of various kinds including a human body without contact with the object at a fast speed. A slit projection method, referred to light cutting method generally, is a representative method thereof. In these methods, an object is imaged by illuminating it with a light for detection, and a three-dimensional image (or distance image) is generated by using the principle of trigonometry. The three-dimensional image is a set of pixels in correspondence to three-dimensional positions of points of the object. The slit light projection method uses a slit light having a linear section as light for detection.
A three-dimensional form model of a whole object can be generated by imaging the object from a plurality of positions and by synthesizing three-dimensional images appropriately. The three-dimensional form model or a part thereof of an object can be used for computer graphics, computer-aided design or the like.
When the above-mentioned active stereo method is used, data has a large amount as large as several tens thousands to several hundred thousands points. If the amount of data is so large, there is a serious burden on handling the three-dimensional form data with a computer, or it takes a long time to display a picture or to operate the data. Thus, processing speed becomes low.
Various techniques have been proposed to reduce the amount of data of three-dimensional form model using polygon representation. For example, it is proposed to delete a vertex from the data in the order of shortness of the distance of a vertex from an average polygon of adjacent polygons until the thinning ratio becomes a specified value (“Decimation of Triangle Meshes”, Computer Graphics, 26, 2, Jul. 1992)). In this method, the three-dimensional form data are thinned out by increasing polygons having shapes similar to a flat plane, so that the density of points is smaller at portions having simple patterns and is larger at portions having complicated patterns.
A user of computer graphics or the like usually wants to process characteristic lines such as edges of an object. Practically, points along characteristic lines become necessary to reproduce a contour of an object for data processing or animation. However, the prior art methods do not necessarily produce a result that a user intended to generate.
In order to solve this problem, a three-dimensional measuring instrument of point contact type may be used to input points one by one along a line intended by a user. However, this technique takes an elaborate work and a long time because the number of input points is as large as several tens thousands to several hundred thousands.